The present invention concerns a procedure for the production of alloys from eutectic alloy systems, preferably Al alloys, in order to form workpieces such as a billet, ingot or slug for forging, cold flow pressing, rolling or extrusion purposes, by, for example, producing an Alxe2x80x94Mgxe2x80x94Si alloy, which can be precipitation-hardened, with 0.35-1.5% weight Mg, 0.35-1.3% weight Si, 0-0.50% weight Fe, 0-1.0% weight Mn, 0-0.05% weight Ti, 0-0.4% weight Cr, 0-0.50% weight Cu with the rest Al and impurities up to a maximum of 0.05% each and 0.15% in total, where the alloy melt is cast to form a workpiece.
In an extrusion plant for the production of aluminium profiles, the aluminium is fed into extrusion devices in the form of workpieces of suitable sizes which are first heated to a suitable temperature which is high enough for extrusion. In general terms, the extrusion devices consist of a cylinder piston device in which the cylinder is provided at one end with a tool in the form of an extrusion die. The aluminium is pressed through the extrusion die with the required cross-section or shape. Alternatively, the workpieces can be fed directly from production to the extrusion plant. In this case the preheating stage will be superfluous.
NO Pat. No. 166 879 discloses a procedure for producing an Al alloy, for example to form an ingot or slug for extrusion purposes, by, for example, producing an Alxe2x80x94Mgxe2x80x94Si alloy, which can be precipitation-hardened, where the alloy melt is cast to form a block or bar, is homogenised and then cooled. The subsequent stages involve the block being heated to a temperature above the solubility temperature of the precipitated secondary particles in the Al matrix and kept at this temperature until the precipitated phases in the Al matrix are dissolved. It is then cooled to the desired extrusion temperature rapidly enough to avoid re-precipitation of the said phases in the alloy structure. Alternatively, the block may be extruded at the said solubility temperature.
This procedure, which will apply to all Al alloys in which local melting arises on account of precipitated phases which are soluble at a higher temperature, represents an improvement regarding extrusion rate, surface quality and strength.
As stated in NO 166 879, the cooling rate, after the alloy has been heated to a temperature above the solubility limit for phases containing, for example, Mg and Si, is decisive for avoiding the precipitation of new, coarse Mgxe2x80x94Si phases. Such phases, together with the surrounding Al matrix, have a lower melting temperature than the rest of the alloy and incipient or preliminary melting (local melting) will, therefore, arise in the phases when the temperature in the metal being processed reaches the melting temperature of the phases together with the surrounding matrix. This will initiate tearing in the metal during processing. By avoiding an agglomeration of phases which have a lower melting temperature than the other parts of the metal, such as the coarse Mgxe2x80x94Si phases, local melting can be avoided, which will mean that the extrusion rate can be increased.
One disadvantage of the said procedure is that if the alloy is cooled too rapidly before extrusion, for example by water cooling, the properties in the finished product may deteriorate, for example reduced mechanical properties after hardening. Furthermore, the actual tensile strength of extrudates from such bars has been found to be highly dependent on the preheating temperature of the bars on account of a temperature-dependent cooling sensitivity in the material.
The above reduction in mechanical properties can be countered by cooling the product after processing (extrudate) at a sufficiently high cooling rate. This usually involves water cooling of extrudates, which entails major disadvantages.
With the present invention, an upper value has been arrived at for the cooling rate in connection with the production of alloys, which is based on new knowledge about the state of the alloy when an excessively high cooling rate is used. If this value is too high, the mechanical properties in the finished product are reduced on account of the above-mentioned increasing cooling sensitivity. Furthermore, a lower, minimum cooling rate has been arrived at, which is sufficient to avoid the undesired precipitation of coarse alloy phases.
The present invention concerns three procedures for producing alloys from eutectic alloy systems, which include an indication of an interval for the cooling rate in connection with the production of the alloys, whereby high productivity (high extrusion rate) and good mechanical properties can be optimised.
The first procedure is used for workpieces which are first teemed and homogenised and which are, for example, stored, transported, etc. before further processing, which workpieces are heated above the solubility temperature of the precipitated phases in the alloy, and the workpiece is cooled to the processing temperature at a cooling rate which is rapid enough to avoid most of the precipitation of the said phases and slow enough to avoid most of the precipitation of dispersoid particles.
The second procedure concerns workpieces which are cast and homogenised, whereby cooling after homogenisation takes place within the same cooling rate interval as in the first precedure.
The third procedure concerns cast workpieces of an alloy, which workpieces are preheated and homogenised in a combined operation, whereby cooling to the processing temperature takes place within the cooling rate interval defined in the first precedure.
In one embodiment the workpiece is cooled, after homogenisation or casting, at a rate which is high enough, for example 55,000xc2x0 C./hour (water cooling) to suppress the precipitation of phases such as particles containing Mgxe2x80x94Si.
The preferred lower and upper limits for the cooling rate are greater than 400xc2x0 C./h and less than 55,000xc2x0 C./h.